Abstract Cardiovascular disease (CVD) kills one in four people annually in the United States and represents a major health concern. Atherosclerosis is the main underlying trigger of CVD and is considered a chronic immune disease, in which antigen-experienced T cells accumulate within atherosclerotic plaques, leading to heart attack and stroke. While plaque destabilization is usually a slow process, diagnosing at-risk individuals often requires invasive methods; thus, there is a growing need to easily identify individuals in danger of heart attack or stroke. Mass cytometry revealed a profound loss of circulating nave CD8 T cells (TN) in subjects with high compared to low CVD risk. Excitingly, subjects with severe CVD showed a strong increase in a CD8+ TN cell population that expressed the memory antigen CD95. Recently, a CD8+ stem cell memory T cell (TSCM) population, denoted by TN cell markers, expressing CD95, and possessing anti-tumorigenic properties, was identified in humans and mice. This CD8+ TSCM population has not been studied in the context of atherosclerosis. This proposal will test the hypothesis that this CD8+ T cell is a potential biomarker of CVD risk and is pro- atherogenic. To test this hypothesis, we will examine whether CD8+ TSCM cell frequencies are increased in the blood of severe-risk individuals within the Multi-Ethnic Study of Atherosclerosis (MESA) cohort and importantly, enhanced with myocardial infarction (MI) via conventional flow cytometry. CyTOF mass cytometry and single cell RNA sequencing (scRNA-seq) will be employed to identify novel CD8+ TSCM markers. We will also mechanistically determine whether human CD8+ TSCM cells possess a pro-atherogenic phenotype by assessing pro-inflammatory cytokine synthesis. To determine if this novel population worsens atherosclerosis, we will examine whether high fat diet feeding elicits elevated CD8+ TSCM cells within atherogenic Apolipoprotein E-/- (ApoE-/-) aorta and aorta-draining para aortic lymph nodes via flow cytometry. Adoptive transfer of CD8+ TSCM cells into immune-deficient ApoE-/- recipients will be performed to investigate whether this novel CD8+ T cell exacerbates atherosclerosis. These results will provide essential insight into how CD8+ TSCM cells contribute to atherosclerosis. Ultimately, we may unearth a novel CD8+ T cell biomarker (CD95 by TN cells) to better predict CVD-related adverse events, potentially improving diagnosis of at-risk individuals, and ultimately, ridding patients of CVD.